A US court ruled Icelandic member of parliament Birgitta Jonsdottir must release details of her Twitter account. Photograph: Halldor Kolbeins/AFP/Getty Images

Icelandic MP and former WikiLeaks volunteer Birgitta Jonsdottir has slammed the decision by US courts to open her Twitter account to the US authorities and is taking her case to the Council of Europe.

On Thursday a US judge ruled Twitter must release the details of her account and those of two other Twitter users linked to WikiLeaks. Jonsdottir learned in January that her Twitter account was under scrutiny from the Justice Department because of her involvement last year with WikiLeaks' release of a video showing a US military helicopter shooting two Reuters reporters in Iraq. She believes the US authorities want to use her information to try and build a case against WikiLeaks founder Julian Assange.

"This is a huge blow for everybody that uses social media," said Jonsdottir. "We have to have the same civil rights online as we have offline. Imagine if the US authorities wanted to do a house search at my home, go through my private papers. There would be a hell of a fight. It's absolutely unacceptable."

She said she would press for the Council of Europe to act on the case, which she believes sets a worrying precedent for private citizens and politicians across the world.

Last month the Inter-Parliamentary Union, which represents MPs from 157 countries, unanimously adopted a resolution condemning the move by the Justice Department. The IPU said the move threatened free speech and suggested it could violate Article 19 of the Universal Declaration of Human Rights, which upholds the right of everyone to freedom of opinion and expression.

"Members of parliament are elected by people to represent them in parliament. In their daily work they legislate and they hold the governments to account. They are unable to perform these duties if they cannot receive and exchange information freely without fear of intimidation," wrote the IPU.

This is the second court victory for the US authorities in a case that has alarmed privacy and free speech advocates; in part because the inquiries' targets might never have known they were being investigated had Twitter not challenged the subpoenas.

The Justice Department also sought the information without a search warrant. US authorities used a 1994 law called the stored communications act to demand that Twitter provide the internet protocol addresses of users, a move that would give the location of the computer they used to log onto the internet. They also asked for bank account details, user names, screen names or other identities, mailing and other addresses.

The petitioners argued that the order suppressed their right to free speech and that their internet protocol addresses should be considered private information. They also argued the demand for information was too broad and unrelated to WikiLeaks.

Judge Liam O'Grady disagreed. In his opinion, "the information sought was clearly material to establishing key facts related to an ongoing investigation and would have assisted a grand jury in conducting an inquiry into the particular matters under investigation."

The Twitter users "voluntarily" turned over the internet protocol addresses when they signed up for an account and relinquished an expectation of privacy, he ruled.

"Petitioners knew or should have known that their IP information was subject to examination by Twitter, so they had a lessened expectation of privacy in that information, particularly in light of their apparent consent to the Twitter terms of service and privacy policy," Judge O'Grady wrote. He also dismissed a petition to unseal the Justice Department's explanation for why it sought the account information.

The fight comes amid widening concerns about online privacy. Facebook is expected to revamp its privacy rules after widespread criticism. Twitter too has become an increasing concern for privacy advocates.

"I want everybody to be fully aware of the rights we apparently forfeit every time we sign one of these user agreements that no one reads," said Jonsdottir.